What is ITBS?

ITBS is:

Excessive friction between your Iliotibial Band and your knee (or hip) bone.
This causes a sharp pain at either the side of your knee or the side of your hip.
Usually caused by increasing your running or biking too fast and/or with bad form.
But is also associated with weight-lifting (standing squats), court sports (raquetball, tennis, handball, etc) and even pregnancy.
Most commonly occurs in one or both knees (see diagram at left), but can also appear at the hip joint. This is not uncommon at pregnancy, where the tendons loosen a bit and the weight distribution shifts. I’ve never heard of any hip-specific exercises or stretches, but think the general ones should help at the hip as well.

Basic definition from the American Running and Fitness Association.

Very complete description of ITBS, the iliotibial band itself, causes, etc.

Medical description related to especially to runners and cyclists, stretches, etc. Says it all, especially if you understand medical terms.

Another good overview for the lay person from The Medical Tent, complete with graphics.

Letter about running knock-kneed.